The effects of hypoxia and hyperoxia on the ventilatory response to impulse and step work loads will be studied in order to determine 1) which component of the ventilatory transient is due to the peripheral chemoreceptors, and 2) if increases in arterial (or end-tidal) CO2 can substitute for peripheral chemoreceptor drive when the latter is altered by breathing hyperoxic gas mixtures in exercise. Further, the effects on the transients in ventilation and heart rate of starting exercise from complete rest and of loadless pedalling at different frequencies will be studied to determine 3) if the rapid components of both parameters are larger and proportional to work load in the rest-to-work transition and 4) if limb motion is itself a stimulus to breathing.